182 VITALISM AND SCHOLASTICISM 



but we will take eight) has two potencies (i.) 

 it may become one-eighth of a single Amphi- 

 oxus; or (ii.) it may become an entire 

 Amphioxus. We shall see the very important 

 bearing of this in a short time. 



Twins a Here it may be said that this observation 

 seems to throw an important light upon the 

 question of twins. Human twin children are 

 of two varieties not sexes in this connection 

 homologous and heterologous. Homologous 

 twins are always of the same sex; practically 

 invariably very like one another in apperance, 

 and are contained in utero in the same bag of 

 membranes. The other variety may be both 

 of the same sex or of different sexes; they are 

 not necessarily any more like one another than 

 members of the same family commonly are, 

 and they have separate bags of membranes. 



It has been thought, and probably correctly, 

 that the homologous twins are the product of 

 a single ovum which has, for some reason still 

 unknown, undergone a division of multiplica- 

 tion before pursuing the ordinary course of a 

 division of development. 



Further, it is probable that the explanation 

 of such curious things as Siamese Twins and 

 Two-headed Nightingales is that the ovum has 

 undergone an imperfect division of multiplica- 

 tion before proceeding on its normal course of 



